This invention relates generally to methods and apparatus for evaluation of fluid flow characteristics and, more specifically, relates to acoustic well logging methods and apparatus for evaluation of flow characteristics of fluid behind subsurface casing or pipe.
When an earth borehole is drilled for the purpose of producing oil, gas, or other fluids, it is common practice to insert casing in the borehole before the production begins. To set the casing cement is pumped into the irregular annulus between the casing and the earth formations. Optimally, the cement bonds both to the casing and to the formations, thereby holding the casing securely in place and also isolating the different strata in the formations from one another. This isolation prevents the vertical migration of fluids from one strata to another and allows production to be obtained fom only those strata or zones desired. If the cement bonding is poor and has vertical fractures or channels, attempts to obtain production from the desired zones can be frustrated because less viscous or higher pressure liquids at other zones will flow vertically through the channel to the production zone. In this situation, hydrocarbon production will be at least substantially effected.
The fluids migrating through vertical channels can be either single-phase gas, single-phase liquid or some multi-phase combinations. The prior art has proposed several methods for locating vertical channels and monitoring fluid movement through these channels, for example U.S. Pat. No. 3,784,828, issued to D. A. Hayes and U.S. Pat. No. 4,173,718, issued to W. H. Fertl and assigned to the assignee of the present invention. However, prior art methods have not been successful in evaluating the nature of fluid flow in these channels; whether the flow is single-phase liquid, single-phase gas or some multi-phase mixture.
Accordingly, the present invention overcomes the deficiencies of the prior art by providing method and apparatus for evaluating the flow characteristics of fluids flowing within channels behind pipe or casing, more particularly for evaluating the flow characteristics of gas and liquid flowing within a channel behind pipe.